A capacitive touch sensor, referred to simply as a touch sensor in the following, may detect the presence and location of a touch or the proximity of an object (such as a user's finger or a stylus) on a surface. Touch sensors are often combined with a display to produce a touch screen. A touch screen enables a user to interact directly with what is displayed on the screen through a graphical user interface (GUI), rather than indirectly with a mouse or touch pad. A touch sensor may be attached to or provided as part of a mobile phone, tablet or laptop computer, for example.
Touch sensors may be classified into grid and matrix types. In a matrix type, an array of electrodes is arranged on the surface which are electrically isolated from each other, so that each electrode in the array provides its own touch signal. A matrix type touch sensor is therefore naturally suited to situations in which an array of touch-sensitive buttons are needed, such as in a control interface, data entry interface or calculator. In a grid type, there are two groups of parallel electrodes, usually referred to as X and Y electrodes, since they are typically arranged orthogonal to each other. A number of nodes are defined by the crossing points of pairs of X and Y electrodes (as viewed in plan view), where the number of nodes is the product of the number of X electrodes and Y electrodes. A grid type touch sensor is the type typically used for touch screens on mobile phones, drawing tablets and so forth. In earlier designs, the X and Y electrodes are arranged either side of a dielectric layer, so they are vertically offset from each other by the thickness of the dielectric layer, vertical meaning orthogonal to the plane of the stack layers. In more recent designs, to reduce stack thickness, the X and Y electrodes are deposited on the same side of a dielectric layer, i.e. in a single layer, with thin films of dielectric material being locally deposited at the cross-overs to avoid shorting between the X and Y electrodes. A single electrode layer design of this kind is disclosed in US 2010/156810 A1, the entire contents of which are incorporated herein by reference.
Touch sensors may also be classified into self capacitance and mutual capacitance types.
In a self capacitance measurement, the capacitance being measured is between an electrode under a dielectric touch panel and the touching finger, stylus etc., or more precisely the effect that the touch's increase in capacitance with the electrode has on charging a measurement capacitor that forms part of the touch IC's measurement circuit. The finger and the electrode can thus be thought of as acting as the plates of a capacitor with the touch panel being the dielectric.
In a mutual capacitance measurement, adjacent pairs of electrodes are arranged under the touch panel, and form the nominal plates of the capacitor. A touching body acts to modify the capacitance associated with the electrode pair by replacing what was the ambient environment, i.e. in most cases air, but possibly water or some other gas or liquid, with the touching object, which may be effectively a dielectric material (e.g. a dry finger, or a plastics stylus) or in some cases could be conductive (e.g. a wet finger, or a metal stylus). One of the pair of electrodes is driven with a drive signal, e.g. with a burst of pulses, and the other electrode of the pair senses the drive signal. The effect of the touch is to attenuate or amplify the drive signal received at the sense electrode, i.e. affects the amount of charge collected at the sense electrode. Changes in the mutual capacitance between a drive electrode and a sense electrode provide the measurement signal. It is noted that in a mutual capacitance grid sensor, there is a convention to label drive electrodes as the X electrodes and sense electrodes as the Y electrodes, although this choice is arbitrary. A perhaps clearer labelling that is often used is to label the drive electrodes as “Tx” for transmission and the sense electrodes as “Rx” for receiver in analogy to telecoms notation, although this labelling is of course specific to mutual capacitance measurements.
Current industry standard touch screens for mobile phones rely on operating the same touch sensor to make both self capacitance and mutual capacitance measurements, since acquiring both is beneficial to gaining additional information about the touch which can be used in post-processing to increase the reliability of interpretation. For example, the presence of moisture can be inferred by comparing mutual capacitance and self capacitance measurements.
Currently, the most common display technologies that are integrated with touch sensors to form a touch screen are thin film transistor (TFT) liquid crystal displays (LCDs) and organic light emitting diode (OLED) displays, and the touch sensor design is a grid design operated to make both self capacitance and mutual capacitance measurements.
The general issue that the present invention addresses is the problems associated with the desire for ever-thinner stacks with ever-higher display resolution for touch screens. Both factors make it increasingly difficult to collect the touch sensor signal, the touch sensing being timed relative to the display cycles to take place during portions of the display cycle where there is the least display noise. In concrete terms, the touch sensing measurement in each display cycle is carried out in a time slot as far away as possible from the horizontal-synchronisation (H-sync) pulse which drives the display pixel rows, when there is the least display noise.
A thinner stack means that electrode layers for the display are closer to the touch sensor electrode layers, so they couple more strongly, which means that display noise becomes bigger for the touch sensor. A thinner stack also means less vertical separation between the two touch sensor electrode layers, so that they have a larger mutual capacitance and so it takes longer to charge and discharge the touch sensor, i.e. the measurements become slower. A charge time on a thicker stack might be 500 ns, whereas the charge time might increase to 1 or 2 μs, or more, for thinner stacks. Higher display resolution means that there are more display rows and columns to address per frame, so that the number of drive pulses which have to be fitted into a 60 Hz refresh rate (i.e. 16.67 ms per refresh) increases. As more time in each refresh is taken up with display drive signals, there is less time available with relatively low display noise that is suitable for collecting the touch sensing signal.
In an OLED (organic light emitting diode) touchscreen, the major coupling between the display electrodes and the overlaid touch sensing electrodes is between the display source (i.e. cathode) layer and the touch sense (Y) layer, since these have conductive features that run parallel to each other with a small vertical separation in the display stack.
It is therefore desirable to provide touch screen designs which allow the touch sensor to operate in combination with the display, as the displays evolve to higher resolution and the touch sensor layers are arranged ever-closer to the display layers.